Two lines of research will be pursued. One will be continuing efforts to account for the differences in size between paramyosin from Limulus (horseshoe crab) and Mercenaria (clam) muscle. The conch (Busycon) paramyosin, which at our present state of research seems to be intermediate in size between the very large Limulus paramyosin and the clam protein, also will be studied. The proposed line of research is an attempt to "create" by enzymatic digestion a paramyosin from the horseshoe crab (or Busycon) which is the same size as the clam. This is preliminary to an anticipated sequencing of all the paramyosins...long into the future. The second line of endeavor is further studies on the Limulus actomyosin ATPase system which seems to have both a Ca ions regulation as in other actomyosin systems and a Mg ions regulation. In some myosin B preparations Mg ions activates the Ca-ATPase activity while in other preparations the Mg ions lowers the Ca-ATPase activity by 50% or more. Although much data has been collected on these two types of preparations, the details seem exceedingly complex and many more experiments will have to be performed before even a clue to the mechanisms can be postulated. BIBLIOGRAPHIC REFERENCES: G.W. de Villafranca & B. Friedman, 1975. Magnesium control of the calcium myosin and actomoyosin ATPase of Limulus polyphemus. Comp. Biochem. Physiol. 51 B; 375 - 383. G.W. de Villafranca, E.H. Foran & C.C. Myers, 1976. Effect of Mg ions on myosin B's CaATPase activity. Biophys. J. 16: 199a (abstract).